Iron and copper corrosion in steam condensate systems results in damage to piping and equipment as well as the loss of high quality water and energy. The corrosion products and process chemicals if returned to the boiler can contribute to the formation of damaging boiler deposits thereby reducing the overall system reliability and increasing operating and maintenance costs.
Iron corrodes in water in the absence of oxygen because iron is less noble than hydrogen. In pure water the ferrous hydroxide (Fe(OH).sub.2) formed by iron and water elevates the pH by providing hydroxide ions and ferrous ions. This reduces the amount of hydrogen ion which tends to retard corrosion. If the water temperature rises, ferrous hydroxide is converted to magnetite (Fe.sub.3 O.sub.4) in the absence of oxygen to form a somewhat protective film barrier. At temperatures of over 120.degree. F., magnetite is formed based upon the overall reaction: EQU 3 Fe+4 H.sub.2 O--Fe.sub.3 O.sub.4 +4H.sub.2.
Thus, under laboratory conditions the corrosion of iron is self limiting. For actual condensate systems however, the presence of contaminants such as dissolved oxygen and carbon dioxide promote the corrosion reaction. In the presence of oxygen, ferrous hydroxide is unstable and ferric hydroxide is formed. Ferric hydroxide is not a corrosion reaction inhibitor as is ferrous hydroxide. Therefore, the presence of free oxygen in a given system enhances the corrosion reaction.
In addition to iron corrosion in water which is augmented by the presence of oxygen, corrosion of copper by oxygen may also occur. Generally, the resulting formation of cuptic oxide is self limiting. If, however, copper complexing agents such as ammonia are present, the copper oxide film cannot become permanently established. Therefore, the presence of ammonia such as from the in-situ decomposition of higher molecular weight amines is undesirable. High concentrations of carbon dioxide in the condensate system, at lower pH values (less than 8) have an effect similar to ammonia in dissolving the copper oxide film.
Carbon dioxide that dissolves in water causes the pH to be depressed and results in the formation of carbonic acid. Carbonic acid promotes the iron corrosion reaction by supplying the reactant H.sup.+. The overall reaction is: EQU 2H.sub.2 CO.sub.3 +Fe--Fe(HCO.sub.3).sub.2 +H.sub.2.
The ferrous bicarbonate is soluble under many conditions and can act as a corrosion reaction retardant. The stability of ferrous bicarbonate in solution is effected by heat, pH and the partial pressure of carbon dioxide above the condensate. Often, these conditions change from location to location within the boiler/condensate system.
In the boiler, sodium carbonate and sodium bicarbonate react with heat plus water to form sodium hydroxide and carbon dioxide. Various external makeup water treatment methods can reduce the potential for carbon dioxide corrosion by lowering the alkalinity of the makeup water.
Due to the aqueous solubility of carbon dioxide, ground waters and surface waters contain carbonates among other dissolved solids. When these waters are heated in steam generating systems, the solubility of carbon dioxide decreases and the gas enters the produced steam. Upon condensation, carbon dioxide again dissolves to form carbonates. Since the condensate contains relatively few dissolved solids and thus little buffering capacity, the weakly acidic carbonate species can drastically lower the condensate pH. In turn, when acidic condensate mixes with makeup water, the steam generator feedwater pH can also decrease.
Carbonate containing waters cause acidic or general corrosion of the iron and copper metallurgies found in condensate and feedwater systems. This type of corrosion is evidenced by a general wastage or by gouging or pitting of the metal surface. If untreated, corrosion can cause failure of condensate return lines, feedwater piping, and other equipment (condensate receivers, pumps, heaters, etc.) associated with steam generator and hot water heating systems.
Several methods have been devised to control acid induced corrosion in these systems. Materials can be added that adsorb to the metal surface to form a thin barrier between the metal and the acidic solution. Examples of effective barrier-forming materials that are routinely used are long chain amines, such as octadecyl amine, and azoles, such as tolyltriazole.
A second, more often utilized method of controlling carbonate-caused corrosion is the addition of amines to neutralize the carbonate and thereby increase the aqueous pH. Many different amines are utilized, but some commonly used materials include cyclohexylamine, morpholine, and methoxypropylamine. On an equal weight basis, the most effective amines are those that possess high basicity and low molecular weight. The high basicity allows attainment of high pH after acid neutralization, and low molecular weight allows greater molar concentrations (and thus more neutralization).
The addition of neutralizing amines neutralizes the acid (H.sup.+) generated by the solution of carbon dioxide in condensate. The amines hydrolyze in water to generate hydroxide ions required for neutralization. By regulating the neutralizing amine feed rate, the condensate pH can be elevated to within a desired range (e.g. 8.5 to 9.0). Numerous amines can be used for condensate pH neutralization and elevation. The selection of the appropriate amine is currently controlled by the basicity, stability and distribution ratio characteristics of the particular amine. Stability of amines fed to a boiler system was considered to be important due to the concern with regard to decomposition into ammonia. The distribution ratio (DR) of an amine is expressed as formula DR equal to amine in vapor phase divided by amine in water phase (condensate) at some defined pressure or temperature.
Amines with a distribution ratio greater than 1.0 have more amine in the vapor phase than the water phase. The distribution ratio is a function of the pressure and the temperature in a boiler/condensate system to be treated. Distribution ratios (at atmospheric pressure}for commonly used neutralizing amines are as follows: Morpholine 0.4; diethylaminoethanol 1.7; dimethylisopropanol amine 1.7; cyclohexylamine 4.0; ammonia 10.0. The varying distribution ratios of commonly used neutralizing amines affect the loss of the amine from the system as well as the area in the system where the amine is most effective. Amines that have low distribution ratios provide excellent pH control at initial condensation sites, but poor neutralization at the final condensation sites. On the other hand, high DR amines are more likely to be found in remote sites in steam that has been in contact with the liquid phase as it passes through the steam distribution system.
In boiler/condensate systems where the bulk of the steam produced is used for turbine supply, morpholine is most suitable or a blend having a high morpholine content. The low DR for morpholine means that morpholine will be present in the initial condensate formed at the wet end of the turbine. In plants with extensive runs of steam lines, a material with a high DR is more desirable. In practice, the best protection is typically provided by a blend of amine products containing a variety of materials with differing distribution ratios.
Typical neutralizing amines have DR's from 0.1 to 10, carbon dioxide has a DR of 100 or more depending upon temperature. Because of this difference in DR's, amines tend to concentrate in the condensate lines closest to the system boiler where as carbon dioxide tends to concentrate in more remote areas of the condensate return system. Thus, conventional amine addition to the boiler feedwater is not sufficient to protect such remote areas from carbon dioxide induced corrosion, often these lines are unprotected or require satellite feed of amines.
Amines having a relatively high volatility compared to the above treatment amines are known. For example, dimethylamine (DMA) trimethylamine (TMA), and diethylamine (DEA) have properties that make them desirable for use in corrosion inhibition in boiler/condensate systems. For example, DMA which has a DR of from 2 to 5, is an extremely strong base (pKa of 10.77) and due to its molecular weight is capable of neutralizing carbonic acid on an approximately 1:1 molar ratio. TMA is between 2-5 times more volatile than cyclohexylamine at boiler pressures from 100 to 1500 psig. DEA has a distribution ratio (at 1000 psig) of 28. Cyclohexylamine is the most volatile neutralizing inhibitor commonly used in the treatment of steam boiler/condensate systems. Thus, it is believed that DMA, TMA, DEA and other low molecular weight amines would be more effective than cyclohexyl amine and other amines used for condensate treatment in following and neutralizing carbon dioxide in the outlying areas of a condensate return system. However, the extreme volatility, i.e. flammability and high atmospheric vapor pressures, of low molecular weight amines has prevented the production of acceptable product formulations containing volatile, low molecular weight amines such as DEA, DMA and TMA for use in boiler/condensate system corrosion treatment.